1. Field of the Invention
The present invention relates to a plastic zoom lens especially suitable for a video camera, an electronic still camera, etc.
2. Description of the Related Art
Two groups of negative and positive zoom lenses are composed of a first group of lenses having negative refracting power arranged on an object side and a second group of lenses having positive refracting power arranged on an image side. Focal lengths of the two groups of negative and positive zoom lenses are changed by changing a distance between the first and second groups of lenses. The two groups of negative and positive zoom lenses provide a relatively small zoom ratio, but the number of constructional lenses is small and a zoom mechanism is simplified. Accordingly, such zoom lenses are widely known and used as lenses for a camera, a video camera, etc.
Recently, a plastic lens has been practically used as a light lens cheaply manufactured. In the above two groups of zoom lenses, the zoom lenses are partially constructed by the plastic lens as shown by e.g., Japanese Patent Application Laying Open (KOKAI) Nos. 57-20713, 57-67908, 58-5707, etc.
However, in the known two groups of zoom lenses, a single plastic lens is used so that no advantages of lightness in weight and cheapness in manufacture of the plastic lens are sufficiently used. Further, as is well known, changes in volume and refractive index of the plastic lens caused by a change in temperature are greater than those of a glass lens so that image forming performance tends to be reduced by the change in temperature. Accordingly, it is necessary to correct the reduction of the image forming performance. However, when only one plastic lens is used, it is very difficult to perform such correction.